Process for trapping gaseous ruthenium on polyvinyl pyridine, more particularly usable for recovering radioactive ruthenium from irradiated nuclear fuels

ABSTRACT

The invention relates to a process fop trapping gaseous ruthenium on polyvinyl pyridine, more particularly usable for recovering radioactive ruthenium from irradiated nuclear fuels. 
     This process consists of contacting a gas containing ruthenium in gaseous form with an adsorbent (11b) constituted by a vinyl pyridine polymer or copolymer for fixing the ruthenium to the latter. The gas can be constituted by yapours from a concentrate of fission products containing ruthenium, which has been heated (at 31) in the presence of an oxidizing agent for volatilizing the ruthenium.

The object of the present invention is a process for the trapping andfixing of ruthenium in gaseous form and in particular rutheniumtetroxide, present in a gaseous flow of an installation for thereprocessing of irradiated nuclear fuels. The irradiation of nuclearfuels in power reactors leads to the production of numerous fissionproducts, whose atomic masses are 70 to 160. These fission productsgenerally appear in the effluents produced at the end of the cycle ofthe fuel. They in particular include metals of the platinum group suchas palladium, rhodium and ruthenium, which are valorizable elements,particularly rhodium due to its use in catalytic converters for cars.Unfortunately, the known processes for the separation of these metalsand such as those described by HAZELTON et al in PNL-5758-UC-70-1986,led to the recovery of the rhodium and ruthenium together, which doesnot make it possible to valorize the rhodium due to the highradioactivity of ruthenium. In addition, ruthenium 106 is one of themain elements contributing to the radioactivity of effluents.

It would therefore be advantageous to have a process for effectivelyseparating the ruthenium from effluents, so as to reduce the activitythereof.

A known ruthenium separation process consists of volatilizing the latterin ruthenium tetroxide form by the oxidation of the ruthenium present innitric solutions using potassium periodate and as is described by BUSHin Platinum Metals Rev, 1991, 35, 4, pp. 202-208. However, this gaseousruthenium must then be recovered, which causes problems which have notbeen solved up to now.

The present invention specifically relates to a process for the trappingand fixing of ruthenium in gaseous form, which makes it possible torecover more than 99% of the ruthenium.

According to the invention, the process for the recovery of theruthenium in gaseous form present in a gas consists of contacting thegas with an adsorbent incorporating a vinyl pyridine polymer orcopolymer and separating the gas from the adsorbent on which theruthenium is fixed.

In this process, the choice of an adsorbent based on a vinyl pyridinepolymer or copolymer makes it possible to trap the ruthenium in gaseousform with very high efficiency, because the ruthenium fixing yieldexceeds 99%.

The gas containing the ruthenium in gaseous form can in particular beconstituted by yapours formed during a volatilization treatment of theruthenium performed on an aqueous effluent containing fission productsfrom the reprocessing of irradiated nuclear fuels.

The invention also relates to a process for the separation of theradioactive ruthenium present in an aqueous effluent containing fissionproducts from the reprocessing of irradiated nuclear fuels and whichcomprises:

a) heating the effluent to a temperature of 100° to 150° C. in thepresence of an oxidizing agent in order to oxidize the ruthenium tovolatile ruthenium tetroxide and

b) recovering the thus volutilized ruthenium by contacting the gascontaining the volutilized ruthenium with an adsorbent incorporating avinyl pyridine polymer or copolymer and separating the gas from theadsorbent on which the ruthenium is fixed.

This procedure for recovering the ruthenium is particularlyadvantageous, because it can be used with numerous effluent types, evenwhen the effluents have high salt contents and which can e.g. extend upto 100 g/1.

In stage a) of the volatilization of ruthenium in ruthenium tetroxideform, it is possible to use as the oxidizing agent alkali metalperiodate or an alkali metal hypochlorite, such as sodium hypochlorite.In this stage, the effluent is generally heated to 100° to 150° C.

The adsorbents usable in the invention are in particular vinyl pyridinepolymers such as poly-4-vinyl pyridine, crosslinked by appropriatecrosslinking agents such as divinylbenzene and tetraethylene glycoldimethacrylate. It is also possible to use copolymers ofvinyl-4-pyridine and divinylbenzene. Preference is given to the use ofcrosslinked polyvinyl-4-pyridine with a grain size of 15 to 60 mesh.

Thus, these crosslinked polyvinyl pyridines have very good rutheniumtetroxide adsorption properties. They also have the advantage of beingstable at temperatures up to 260° C. under atmospheric pressure and ofresisting reducing and oxidizing agents, so that they can be used in thepresence of gases containing constituents such as nitrous yapours,oxygen, chlorine and water vapour. Moreover, they are not sensitive toirradiation, because no degradation is observed after irradiating for 8hours 24 minutes using a cesium 137 source producing a dose rate of 2Mrad/h, i.e. 1.05·10¹⁶ MeV/kg.

The polyvinyl-4-pyridine used in preferred manner in the invention has aglass transition point of 151° C., which is not modified by irradiation.It can also be used in the form of a powder having an appropriate grainsize.

For performing the process according to the invention, it is possible inparticular to filter the gas containing the ruthenium in gaseous formthrough a powder of the adsorbent.

Preferably, working takes place at a temperature above ambienttemperature, e.g. at a temperature of the gas on entering the filter of53° to 63° C.

The adsorbent quantity used is dependent on the ruthenium quantity to beextracted and generally 0.07 to 0.5 g of polyvinyl pyridine are used for1 mCi of Ru.

After fixing the .ruthenium to the polyvinyl pyridine, it is possible torecover the latter in aqueous solution, if desired, by treating thepolyvinyl pyridine in an appropriate aqueous solution for dissolving theruthenium in it. For example, such a solution can be constituted by asulphuric acid solution.

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein show:

FIG. 1 An installation for filtering a ruthenium-containing gas.

FIG. 2 An installation for the recovery in an aqueous solution of theruthenium fixed to the adsorbent of the installation of FIG. 1.

FIG. 1 shows an installation making it possible to volatilize and thentrap the ruthenium present in an aqueous solution of fission products.

This installation comprises a reactor (1) able to receive the solutionof fission products, which is associated with an oxidizing agentdistributor (3) by a pipe equipped with a valve (5). The reactor (1) canbe heated by a heating means (7) and the yapours given off are suppliedby a pipe (9) to a filter (11) which traps the ruthenium. The lattercomprises a sintered product (11a), above which is placed the polyvinylpyridine powder (11b), which is in turn covered by glass wool (11c).

The vapour filtered in the filter (11) is collected in a container (13)and then placed in a safety bottle (15) filled with a soda solution bymeans of a pipe (17) equipped with a non-return valve (19) and a gasdiffuser (21) immersed in the soda solution. The gas bottle (1S) islinked with a venturi (23) making it possible to form a vacuum in theinstallation.

In order to make the said installation function, into the reactor (1) isintroduced an aqueous solution of fission products constituted by aconcentrate of fission products and this solution is heated to 100° to150° C. using the heating means (7) after introducing into it sodiumhypochlorite from the distributor (3).

Under these conditions, the ruthenium is oxidized by the sodiumhypochlorite into ruthenium tetroxide, which is volatilized and broughtby the pipe (9) together with the gas formed by the yapours from thesolution incorporating NO₂, Ci₂, H₂ O and O₂, to the ruthenium filter(11). The temperature of the gas on entering the filter is 58°±5° C.After passing onto the filter, the gas is collected in the container(13) and then in the 1N soda-filled safety bottle (15), in whichdissolves the ruthenium not fixed to the filter.

The ruthenium filter is formed by a vertically positioned, diameter 30mm glass tube and having a sintered product (11a) on which are deposited3 g of crosslinked 4-polyvinyl-pyridine having a grain size of 60 meshand a crosslinking level of 2% and held by glass wool (11c). In thesafety bottle (15) recovery takes place of the distillate and theruthenium not fixed to the filter.

In order to determine the ruthenium quantity trapped on the filter, thepolyvinyl pyridine powder of the filter (11) is dissolved in sulphuricacid using the installation shown in FIG. 2.

This installation comprises a reactor (31) associated with a sulphuricacid distributor (33) by a pipe equipped with a valve (35) and heatingmeans (37). The yapours given off in the reactor (31) can be brought bya pipe (39) into a container (41) connected by a pipe (43) equipped witha non-return valve (45) and a diffuser (46) to a safety bottle (47)containing soda. This safety bottle is associated by a pipe (49) with aventuri (51) for producing a vacuum in the installation.

For recovering in solution the ruthenium fixed to the filter, theruthenium-containing polyvinyl pyridine powder is introduced into thereactor (31) and to it is added concentrated sulphuric acid, followed byheating. During the heating operation, the polyvinyl pyridine isdissolved in the sulphuric acid at the same time as the ruthenium whichis not volatile in this medium and the optionally volatilized rutheniumis recovered in the safety bottle (47), where it dissolves in the 1Nsoda.

This is followed by the analysis of the solutions obtained in the safetybottle (15) and the safety bottle (47) by gamma spectrometry in order todetermine their ruthenium contents. This analysis method gives anaccuracy of 2% for activities above 500 mCi/l, 5% between 1 and 500mCi/l and 10% for activities below 1 mCi/l.

The following examples illustrate the results obtained during theprocessing of concentrates of fission products.

EXAMPLE 1

In this example, treatment took place in the manner describedhereinbefore of a concentrate of fission products incorporating 49.8Ci/l of cerium 144, 49.8 Ci/l of praseodymium 144, 35.7 Ci/l ofruthenium 106, 49.4 Ci/l of cesium 137 and 6.4 Ci/l of cesium 134, using2 ml of concentrate (71.4 mCi of Ru 106) to which were added 60 ml ofNaC10 and whilst heating to evaporation for 1 h.

At the end of the operation 33.268 mCi of Ru remained in the reactor and38.07 mCi of Ru were collected on the filter (11) and 0.062 mCi ofruthenium in the safety bottle (15).

Thus, there was a ruthenium fixing yield of 99.8% and a ruthenium fixingrate of 12.7 mCi of Ru 106 per gramme of polyvinyl pyridine.

EXAMPLE 2

The same operating procedure as in example 1 was followed for treating 2ml of concentrate of fission products incorporating 21 Ci/l of Ce 144,21 Ci/l of Pr 144, 16.5 Ci/l of Ru 106 (33 mCi), 37.5 Ci/l of Cs 137,4.4 Ci/l of Cs 134 and 0.7 Ci/l of Eu 154, carrying out heating for only20 min. and using 18 ml of NaC10. At the end of the operation there were24 mCi of Ru in the reactor and 8.2 mCi of ruthenium were collected onthe filter (11) and 0.0396 mCi of ruthenium in the safety bottle (15).

Thus, there was a ruthenium fixing yield of 99.5% and a Ru 106 fixingrate of 2.73 mCi/g of polyvinyl pyridine.

The results obtained in these examples demonstrate that the use ofpolyvinyl pyridine for trapping ruthenium in gaseous form makes itpossible to achieve very high fixing deficiencies.

EXAMPLE 3

In this example the ruthenium-charged filter of example 1 was treatedfor the recovery of ruthenium fixed in an aqueous sulphuric solution andwhilst using the installation of FIG. 2.

To this end introduction took place into the reactor (31) of thepolyvinyl pyridine powder of the filter of example 1 with 30 ml ofconcentrated H₂ SO₄, followed by heating to 150° C. Thus, PVC andruthenium were dissolved in sulphuric acid, because Ru is not volatilein a sulphuric medium. The yapours produced are sucked into the safetybottle (47) filled with soda in order to collect there the rutheniumwhich might possibly be volatilized. At the end of the operation, the Rucontent of the sulphuric solution and the soda solution is determined bygamma spectromerry.

The following results were obtained:

sulphuric solution of the reactor (31): 38 mCi of Ru 106,

soda solution of the safety bottle (47): 0.0736 mCi of Ru 106.

The ruthenium recovery rate in the sulphuric solution is consequentlyvery high, i.e. 99.8%.

We claim:
 1. Process for the recovery of ruthenium in gaseous formpresent in a gas, characterized in that it consists of contacting thegas with an adsorbent incorporating vinyl pyridine polymer or copolymerand separating the gas from the adsorbent on which the ruthenium isfixed.
 2. Process for the separation of radioactive ruthenium present inan aqueous effluent containing fission products resulting from thereprocessing of irradiated nuclear fuels, characterized in that itcomprisesa) heating the effluent to a temperature of 100° to 150° C. inthe presence of an oxidizing agent for oxidizing the ruthenium tovolatile ruthenium tetroxide and b) recovering the thus volatilizedruthenium by contacting a gas containing the volatilized ruthenium withan adsorbent incorporating a vinyl pyridine polymer or copolymer andseparating the gas from the adsorbent on which the ruthenium is fixed.3. Process according to claim 2, characterized in that the oxidizingagent is sodium hypochlorite.
 4. Process according to either of theclaims 1 and 2, characterized in that heating takes place in vacuo. 5.Process according to either of the claims 1 and 2, characterized in thatthe absorbent is constituted by crosslinked poly-4-vinyl pyridine. 6.Process according to either of the claims 1 and 2, characterized in thatthe contacting and separation of the gas with the adsorbent are carriedout by filtering the gas through a powder of said adsorbent.
 7. Processaccording to either of the claims 1 and 2, characterized in that the gasalso comprises at least one constituent chosen from among nitrousyapours, oxygen, chlorine and water vapour.
 8. Process according toeither of the claims 1 and 2, characterized in that the contacting ofthe gas with the adsorbent takes place at a temperature of 53° to 63° C.9. Process according to either of the claims 1 and 2, characterized inthat the adsorbent is then treated to dissolve the ruthenium fixed tothe adsorbent in an aqueous solution.
 10. Process according to claim 9,characterized in that the aqueous solution is a sulphuric acid solution.